This invention relates generally to high pressure, high power, continuous wave (CW) electro-dynamic lasers, and, more particularly, to a CW electro-dynamic laser having an acoustic absorbing electrode incorporated therein.
Since the development of the first working lasers, considerable time and effort has been expended in the search for high power output laser systems. The possible applications of high power lasers are unlimited in the fields of communication, manufacturing, construction, medicine, space exploration, and defense.
The gas laser has grown out of the initial laser effort and is representative of one of the more sophisticated laser techniques which has the capability of providing very high power radiation output, the primarily to the large gas handling capability characteristic of such a system and due to the large quantity of energy which can be added to the gases flowing in such systems.
While the preferred embodiment of the present invention will be described in connection with an e-beam ionized, electrically excited nitrogen (N.sub.2), carbon dioxide (CO.sub.2) and helium (He) laser, it may be applied to other systems where a flowing laser gas is required or useful and including, but not restrictive to, gas constituents other than N.sub.2, CO.sub.2 and He as well as other lasing systems. For an electron beam ionized laser, the discharge produced does not require ionization by the discharge electrons, in a lasing environment, hence the electrical discharge can be adjusted to the correct electron temperature for most efficient laser operation. Moreover, the laser is volumetric in the sense that the proper gas temperature and lower laser state concentrations are maintained by volumetric flow, instead of diffusion through the gas to cooled side walls. Further, the laser may be operated in the static pulse as well as the flowing gas mode.
Two conditions must be fulfilled in order to bring about laser action: (1) population inversion must be achieved and (2) a process of photon amplification must be established in a suitable cavity or resonator such as, for example, an optical cavity, optical resonator or resonant cavity. Population inversion can, for example, be accomplished if (1) the atomic system has a least three levels (one ground and at least two excited levels) which can be involved in the excitation and emission processes and (2) the lifetime of one of the most energetic of the excited states is much longer than that of the other or others.
When a system is in a condition where light (photon) amplification is possible, laser action can be achieved by providing (1) means for stimulating photon emission from the long-lived state, and (2) means for causing photon amplification to build up to extremely high values. In the usual embodiment, this is accomplished by fashioning the medium containing the active atoms into a chamber with highly (as far as possible) reflecting ends polished so highly that the surface roughness is measured in terms of the wave length of the laser. The ends may be simply polished metal or they may be silvered or dielectric coated so that they behave as mirrors which reflect photons coming toward them from the interior of the chamber. Such a structure, whether the mirrors are within or outside the chamber, is called the optical or resonant cavity. If now pumping means, such as for example, an electric discharge acts on the medium and brings about population inversion of the long-lived state with respect to another lower energy excited state even though the long-lived state is only relatively long-lived, in a small fraction of a second there will be spontaneous emission of photons. Most of these photons will be lost to the medium but some of them will travel perpendicular to the ends and be reflected back and forth many times by the mirrors. As these photons traverse the active medium, they stimulate emission of photons from all atoms in the long-lived state which they encounter. In this way the degree of light amplification in the medium increases extraordinarily and because the photons produced by stimulated emission have the same direction and phase as those which stimulate them, and assuming the optical quality of the laser media is suitable, the electromagnetic radiation field inside the chamber or cavity is coherent. In order to extract a useful beam of this coherent light from the cavity, one (or both of the mirrors is made partially transmissive. A portion of the highly intense beam leaks by the mirror, and emerges with regularly spaced wave fronts. This is the laser beam.
In the electro-dynamic laser an electron beam is fired into a gas filled optical or resonant cavity so as to ionize a fraction of the gas to provide free electrons. The use of an electron beam for electric laser pumping is fully described in U.S. Pat. No. 3,702,973 issued Nov. 14, 1972. These electrons are subject to the sustainer voltage which adds energy to them, heating them to a desired temperature. In the case of the CO.sub.2 laser, the electrons transfer some of their energy to N.sub.2 and CO.sub.2 in the cavity by collision processes, pumping (quantum mechanically) these gases to an upper laser energy level. The N.sub.2 transfers its vibrationally excited energy to the CO.sub.2. The CO.sub.2 relaxes to a lower level by the emission of radiation. The cavity is bounded with mirrors which reflect some of the stimulated emission back into the cavity stimulating more emission, etc. The radiation is eventually led out of the cavity in the form of a laser beam. Because not all of the energy introduced is discharged by the gas in the form of laser radiation output energy, both the processor of heating the electrons and lasing heat the gas as well. This heating of the gas has two (or more) deleterious effects, (1) it can fill up the lower lasing level which causes lasing to stop, and (2) it can cause the temperature of the gas to rise to the point where the components themselves are damaged. In order to prevent such an undesirable occurrence the laser gas is caused to flow through the cavity in a wind tunnel-type arrangement so as to replace the hot gas with fresh cool gas.
The electro-dynamic laser cavities have optical resonator modes and acoustic resonator modes. These modes or the electromagnetic and acoustic waves, can and do interact on each other through the laser medium with the growth of one causing a corresponding growth of the other which in turn causes more growth of the first, etc. This is referred to as a mode-medium interaction instability. Somewhat oversimplifying, the electromagnetic energy losses in the resonant optical cavity are sensitive to small perturbations of the gas density profile of the gas in the resonant cavity (the density profile initially exists because of the standing acoustic waves) which are the Fourier decomposition at acoustic noise and the density profile in turn is affected by variations in the gas flux. This is a positive feedback process in which under proper circumstances a perturbation of the flux causes a change in the density profile of the gas which in turn increases the magnitude of the perturbation of the flux which then magnifies the change in the density profile until the growth of one or both of these effects causes lasing action to cease.
As a consequence thereof, the power output of high pressure, high power, continuous wave (CW) electro-dynamic lasers modulates severely. This modulation varies, depending on particular operating conditions of the laser, from .+-.10% about a mean CW component, to full off and on, 100%. Further, this modulation could under certain circumstances substantially reduce electrical efficiencies and degrade output beam quality to unacceptable levels. Since maintaining good optical quality or homogeneity of the laser medium is of prime importance in the development of high-power high-performance lasers, alleviating the problems set forth above are considered essential in furthering the development of CW electro-dynamic laser technology.